init: add an init_chroot helper

Add a simple helper to chroot with a kernel space file name and switch
the early init code over to it.  Remove the now unused ksys_chroot.

Signed-off-by: Christoph Hellwig <hch@lst.de>

init/do_mounts.c

@@ -629,7 +629,7 @@ void __init prepare_namespace(void)
 out:
 	devtmpfs_mount();
 	init_mount(".", "/", NULL, MS_MOVE, NULL);
-	ksys_chroot(".");
+	init_chroot(".");
 }
 
 static bool is_tmpfs;

init/do_mounts_initrd.c

@@ -63,7 +63,7 @@ static int __init init_linuxrc(struct subprocess_info *info, struct cred *new)
 	/* move initrd over / and chdir/chroot in initrd root */
 	init_chdir("/root");
 	init_mount(".", "/", NULL, MS_MOVE, NULL);
-	ksys_chroot(".");
+	init_chroot(".");
 	ksys_setsid();
 	return 0;
 }
@@ -101,7 +101,7 @@ static void __init handle_initrd(void)
 	/* move initrd to rootfs' /old */
 	init_mount("..", ".", NULL, MS_MOVE, NULL);
 	/* switch root and cwd back to / of rootfs */
-	ksys_chroot("..");
+	init_chroot("..");
 
 	if (new_decode_dev(real_root_dev) == Root_RAM0) {
 		init_chdir("/old");